The present invention pertains to calender stacks used in the paper manufacturing process, and, in particular, to a process for making chilled iron rolls utilized in calender stacks.
Paper is typically formed as a continuous web. One process step performed on the paper web after the web has dried involves a calender stack. A calender stack includes a pair of aligned, counter-rotating chilled iron rolls having face lengths slightly longer than the width of the web and which press together along the face lengths to form a nip therebetween. As the paper web passes through the nip, the rotating rolls impart a smooth finish to the paper web, which enhances its printability, as well as densify and impart a generally consistent sheet thickness to the paper web.
Multiple shapes and types of calender stack rolls are known in the art. For example, the paper contacting portion of the rolls can be provided by rolls cylindrical in shape or cylindrical with a slight crown along the face length to account for roll deflection during use. In addition, some rolls are substantially solid throughout their diameters, while other rolls are formed with an exterior outer shell defining an interior hollow that accounts for much of the roll diameter. For all these calender stack roll designs, a shared characteristic is a very hard exterior surface or circumference formed of chilled iron which is necessary to give the rolls acceptable wear life due to the abrasive properties of paper. To prepare the rolls for use, the chilled iron exterior is ground very smooth, giving the rolls a shiny finish. Over time, the paper abrasiveness wears down the rolls, giving the rolls a matte finish. When worn, the rolls in some circumstances can have their exteriors reground to reacquire the smooth, paper contacting periphery.
One way of manufacturing chilled iron rolls such that they have the hard, chilled iron exterior surface involves the process step of pouring hot molten metal into a vertically aligned mold. In particular, a roll mold, extending upwards from the foundry floor or from the floor of a specially formed foundry pit over thirty feet or more in order to form a roll face length in the typical range of 90 to 400 inches, is statically positioned on end. The outside diameter of the mold is then lined with chill blocks made of iron and at room temperature, which serve as heatsinks during casting. When hot molten metal is poured under gravity into the mold from above, the metal along the outside diameter surface of the mold thermally chills and solidifies rapidly as heat is drawn off by the chill blocks. The outer diameter metal of the roll is thereby transformed into very hard white or chilled iron. For example, when a typical metal composition used to make gray iron is poured into the vertically situated mold, the gray iron contacting the mold outer diameter wall thermally chilled by the chill blocks quickly freezes. The time/temperature/transformation properties of gray iron result in the outer gray iron transforming into white iron. The resulting microstructure of the white iron is essentially iron carbide and can be ground to form the paper contacting roll periphery.
A significant shortcoming of this method is a result of the vertical static casting. In particular, the mold must be carefully balanced on end during casting and surrounded by chill blocks which increases the expense of the casting process. Moreover, not all foundries are readily capable of lifting and pouring the molten metal into the top of the mold as high above the foundry floor as typically is the mold top.
Another way of manufacturing calender stack rolls such that they have chilled iron exterior surfaces is with a centrifugal mold. The mold for forming the calender stack roll is horizontally aligned and rotated about its longitudinal axis. During rotation, hot molten metal such as is typical for gray iron chemistry is poured into the mold, and the centrifugal force causes the metal to coat the inner surfaces of the mold. In centrifugal moldings, chill blocks are not typically utilized, but instead an element such as chromium is added to the poured gray iron chemistry to form a chemically chilled white iron which includes chromium carbides. Once the outer diameter of the roll has been formed with the chemically chilled white iron having a high hardness, the remainder of the roll may be formed with a gray iron chemistry lacking the chromium.
Shortcomings of this technique of forming chilled iron rolls include the need to spin a lengthy roll mold at a relatively high speed to achieve the centrifugal casting. In addition, the metal must be poured such that it fills the full face length of the mold to a uniform thickness.
Another problem with existing calender stack roll technology relates to the practical hardnesses of the chilled iron provided on the paper contacting roll periphery. While higher hardnesses are desirable from the standpoint of providing a longer wearing roll, obtaining higher hardnesses is hindered by the process of thermally relieving those residual stresses associated with roll formation. In particular, unless the residual stresses in the gray and white type irons in the rolls resulting from the casting of the rolls are reduced, it is possible under some circumstances for the rolls to fail or possibly explode after solidification and in use. Therefore, stress relief techniques known in the art, such as bringing the temperature of a part to a high level and holding the temperature for a selected period of time, are ordinarily used on the rolls after casting. However, these techniques have an adverse or lowering effect on the hardness of the already formed chilled iron exterior, and therefore the industry presently typically furnishes rolls with these exterior surfaces having a hardness of from around 68 to 72 according to the Shore scleroscope hardness test.
Another problem with rolls formed using the above techniques is their limited service life. The use of chill blocks achieves a finite radial thickness or depth of chilled iron having the high hardness, carbidic material. This thickness is typically between about 0.5 inch to about 0.75 inch. During the lifetime of the roll, the cycles of wear from the paper and subsequent regrindings remove the harder peripheral layer until the lower hardness gray iron is reached, at which time the roll is no longer serviceable and an expensive replacement is required. Thus, it is desirable to provide a chilled iron roll for a calender stack which does not suffer from some of these shortcomings.